This achievement is unlocked mid-mission after completing the second mission objective at the very first outpost. The objective has you "Engage hostiles". Shoot all targets in the base and it will pop once you destroy nearly all of them just before more targets appear and start to flee.
To play local Co-op you need a second controller (or alternative input device such as music game peripherals). When selecting the mission and difficulty you can change the game mode from Single player to local Co-op. One player will fly while the other shoots.
Please note that only the pilot, the player who pushed
first and whose save game got loaded, unlocks the achievement!Review 23 September 2014: Added button icon.